I'm not a fan of adding dependency that some users may not need as it bloats the lib size. Can we remove the dep and make this feature be optional?
I think only forwarding metadata when Reflect metadata is supported would work.

That's reasonable, I'll fix it.

Why do we need such temporary object and to process for it in different functions? Why don't we just directly assign the original metadata to component properties?

Do you mean ComponentOptions? If you mean that, it's a tricky way because i don't want to change options interface and expose internal work of Component decorator.

No, I don't mean that.
I mean why we need to store metadata in temporary object rather than just forward them directly on the place.
https://github.com/vuejs/vue-class-component/pull/227/files#diff-7db1d7c8dc88ce00d1aa236fa0c128ccR40
https://github.com/vuejs/vue-class-component/pull/227/files#diff-7db1d7c8dc88ce00d1aa236fa0c128ccR118

Because forwarding code will be duplicated. If you okay with that, I'll do it in the way you are asking. Also, we don't have a new class object until almost the end of function.

Well, then we can extract the logic as a function. I believe we can remove reflectionMap without duplication.

something like:

function forwardMetadata(to: typeof Vue, from: typeof Vue, propertyKey: string): void {
  Reflect.getOwnMetadataKeys(from, propertyKey).forEach(metaKey => {
    const metadata = Reflect.getOwnMetadata(metaKey, from, propertyKey)
    Reflect.defineMetadata(metaKey, metadata, to, propertyKey)
  })
}

Then we just call it inline:

  Object.getOwnPropertyNames(Original).forEach(key => {
     // `prototype` should not be overwritten
     if (key === 'prototype') {
       return
     }
 	 
-    reflectionMap.static[key] = Reflect.getOwnMetadataKeys(Original, key);
+    forwardMetadata(Extended, Original, key)

     // Some browsers does not allow reconfigure built-in properties
     const extendedDescriptor = Object.getOwnPropertyDescriptor(Extended, key)
     if (extendedDescriptor && !extendedDescriptor.configurable) {

  forwardStaticMembersAndCollectReflection(Extended, Component, Super, reflectionMap)
-  copyReflectionMetadata(Component, Extended, reflectionMap)
+  Object.getOwnPropertyNames(proto).forEach(key => {
+    forwardMetadata(Extended.prototype, proto, key)
+  })

I think we don't need this if we directly forward metadata and remove reflectionMap.